1. Field of the Invention
The present invention relates to an integration type photovoltaic apparatus using an amorphous silicon related material film and a method of fabricating the same.
2. Description of the Prior Art
In recent years, a photovoltaic apparatus using an amorphous silicon related material film as a photoactive layer has been employed for a variety of applications. This owes much to the development of an integration type amorphous silicon (a-Si) photovoltaic apparatus so adapted that a high voltage is taken out by making cascade connection of a lot of photoelectric converting elements on one substrate.
A general a-Si photovoltaic apparatus is formed by laminating on a glass substrate a transparent conductive film, p-type, i-type and n-type a-Si films, and a back metal electrode film in this order. In the integration type a-Si photovoltaic apparatus, cascade connection of a lot of photoelectric converting elements is made such that a high voltage is taken out of one substrate as a whole.
In order to form an integration type structure; the transparent conductive film, the a-Si film, and the metal electrode film on the glass substrate must be separated from each other. As a method of separating the respective films, a laser patterning method using a laser has been mainly used (see U.S. Pat. No. 4,650,524, for example).
A conventional method of fabricating an integration type photovoltaic apparatus using a laser patterning method will be described in accordance with FIGS. 5A to 5E. FIGS. 5A to 5E are enlarged cross-sectional views of a principal part showing the steps of the conventional method of fabricating an integration type photovoltaic apparatus, each illustrating as its center an adjacent spacing portion where two photoelectric converting elements are electrically connected in series.
A transparent conductive film 102 composed of ITO (In.sub.2 Sn.sub.2 O.sub.3), SnO.sub.2, etc. is formed on one main surface of a light transmissive insulation substrate 101 composed of glass, etc. The transparent conductive film 102 is divided in a strip shape into an arbitrary number of stages by irradiation of a laser beam, for example (see FIG. 5A). An amorphous silicon related material film 103 composed of a-Si having a pin junction inside thereof is deposited on the divided transparent conductive film 102 (see FIG. 5B).
Thereafter, a laser beam is irradiated along a dividing line of the transparent conductive film 102 such that it is not overlapped with the dividing line from the other main surface of the substrate 101, to radially discharge hydrogen inside the amorphous silicon related material film 103, and the amorphous silicon related material film 103 is removed by the discharge of hydrogen, to divide the amorphous silicon related material film 103 (see FIG. 5C).
A back metal electrode film 104 composed of aluminum, etc. is then formed on the amorphous silicon related material film 103, to connect the transparent conductive film 102 and the back metal electrode film 104 (see FIG. 5D). Thereafter, a laser beam is irradiated along the dividing lines of the transparent conductive film 102 and the amorphous silicon related material film 103 such that it is not overlapped with both the dividing lines from the other main surface of the substrate 101, to rapidly discharge hydrogen inside the amorphous silicon related material film 103, and the amorphous silicon related material film 103 and the back metal electrode film 104 formed thereon are removed by the discharge of hydrogen, to separate adjacent cells (see FIG. 5E).
As the thickness of the amorphous silicon related material film is optimized with measures to prevent degradation by light and improvement in conversion efficiency, and a lamination type photovoltaic apparatus using a narrowband gap material is developed, the amorphous silicon related material film is thinned.
As the amorphous silicon related material film is thinned, the absolute amount of hydrogen inside the amorphous silicon related material film is insufficient in patterning the back metal electrode film, so that the back metal electrode film is not completely removed. Particularly when the thickness of the amorphous silicon related material film is not more than 4000 angstroms, insufficient processing due to lack of the absolute amount of hydrogen inside the amorphous silicon related material film becomes significant. That is, as shown in FIGS. 6A and 6B, a residue 105 is left at an end of a removed part of the back metal electrode film, so that sufficient characteristics cannot be obtained. FIG. 6A is a cross-sectional view of a principal part showing a separating portion between cells, and FIG. 6B is a schematic view showing an optical microscopic image in the separating portion between cells.
On the other hand, a method of thinning the back metal electrode film is considered in order that the amorphous silicon related material film and the back metal electrode film can be removed even by a small dynamic function. If the thickness of the back metal electrode film is decreased, however, the heat radiation effect due to heat conduction in irradiating a laser beam is reduced, so that the end of the back metal electrode film is thermally deformed, and a fused scattered material adheres to the removed part of the back metal electrode film again, for example.
The results of simulation of the temperature distribution in the direction of depth in a case where aluminum (Al), for example, is used as a material of the back metal electrode film, and a Nd:YAG laser is irradiated at an energy density of 5.times.10.sup.7 W/cm.sup.2 from an amorphous silicon semiconductor layer are shown in FIGS. 7 and 8. FIGS. 7 and 8 are diagrams respectively showing the results of simulation of the temperature distribution in the direction of depth in a case where the thickness of the back metal electrode film is 6000 angstroms and the results of simulation of the temperature distribution in the direction of depth in a case where the thickness of the back metal electrode film is 3000 angstroms. These drawings indicate that if the thickness of the back metal electrode film is decreased from 6000 angstroms to 3000 angstroms, the whole of the back metal electrode film is completely fused in excess of its melting point. As a result, the end of the back metal electrode film warps upward, and the fused scattered portion adheres to the inside of a separating groove again, as shown in FIGS. 9A and 9B. FIG. 9A is a cross-sectional view of a principal part showing a separating portion between cells in a case where the back metal electrode film having a thickness of 3000 angstroms is used, and FIG. 9B is a schematic view showing an optical microscopic image in the separating portion between cells.
As a result, the reliability of the separating portion between cells in the back metal electrode film is degraded, as shown in FIG. 10. When an integration type photovoltaic apparatus comprised of 12 stages is formed, an open voltage varies more greatly, as compared with an open voltage of a cell having a small area (hereinafter referred to as a reference 1 cm square) which does not require separation by a laser beam. This is the result of short caused by the contact of the warping, for example, at an end of the separating portion of the back metal electrode film with the other end.